The present invention relates generally to microelectromechanical systems (MEMS), and more particularly to a thin film membrane structure and methods of making such membranes.
Developers of microelectromechanical systems (MEMS) often wish to produce device designs that require the fabrication of completely closed hollow structures. These hollow structures are difficult to realize using traditional micromachining processes, because in order to create a hollow cavity by the removal of a sacrificial layer, access holes must be left by which an etchant can reach the sacrificial material. If these holes are not to compromise the structure, they must be placed far apart, making it necessary to etch for a long time to remove the sacrificial material.
For structures such as sealed vacuum cavities and immersible ultrasonic transducers, these etch holes must be sealed by a subsequent material deposition. This requirement precludes the use of standard etch windows, instead requiring the fabrication of very thin slits that can be more readily sealed. This complicates the fabrication process and places restrictions on the device design. The sealing deposition itself can deposit unwanted material inside the hollow spaces, degrading internal structures.
It is known that thin layers of polycrystalline silicon (polysilicon) can sometimes be permeable to hydrogen fluoride (HF) etching solutions, raising the possibility of simply removing a sacrificial oxide layer through a layer of overlying polysilicon. The use of permeable polysilicon etch windows have been useful in creating hollow vacuum cavities. The HF resistance of thin polysilicon films used for MOSFET gates has also been investigated. Some studies have been done to determine the process conditions required to obtain permeable polysilicon films, and to examine the microstructure of some permeable films.
Unfortunately, MEMS researchers attempting to use permeable polysilicon have had difficulty obtaining repeatable results. The recipes used heretofore vary widely, and often yield impermeable films. A particular recipe may work at certain times and not at others. Further, the physical origin of the film permeability has remained unclear. If the processing of permeable polysilicon films were better understood, the use of such films could greatly simplify complicated process flows and new device designs might become practical.
In one aspect, the invention features a membrane structure comprising a silicon film having a grain structure including grains defining pores therebetween.
In another aspect, the invention is directed to a membrane structure comprising a silicon film including grains having gaps formed therebetween to define individual pores. The maximum cross-sectional dimension of any one grain is approximately equal to the thickness of the film.
Various implementations of the invention may include one or more of the following features. The lateral dimension of any pore is less than that of any grain. The lateral dimension of the pores is between about 10 and 50 nanometers (nm). The thickness of the film is less than or equal to about 150 nm. The thickness of the film is between about 50 and 150 nm. The roughness of the film is approximately equal to its thickness. The film forms a filter. The film is conformal to an underlying surface. A structural layer is provided to support the film. A conformal layer is formed on the film to provide a selected chemical or biological function.
In yet another aspect, the invention is directed to a membrane filter structure comprising a silicon film having a grain structure including grains defining pores therebetween. A lateral dimension of the pores is between about 10 and 50 nm and the maximum diameter of any one grain does not exceed the thickness of the film.
In still another aspect, the invention features a method of fabricating a membrane structure. The method comprises forming a sacrificial layer over a first surface of a substrate and forming a silicon layer over the sacrificial layer. The silicon layer is formed such that it has a grain structure including grains defining pores therebetween wherein the maximum diameter of any one grain does not exceed the thickness of the membrane structure. The sacrificial layer is removed.
Various implementations of the invention may include one or more of the following features. The method may further include forming a passageway through the substrate. A conformal layer may be formed over the silicon layer to provide a selected chemical or biological function.
In still another aspect, the invention is directed to a method of fabricating a membrane structure comprising forming a sacrificial layer over a surface of a substrate. A structural layer is formed over the sacrificial layer and a silicon layer is formed over the structural layer. The silicon layer is formed such that it has a grain structure including grains defining pores therebetween wherein the maximum diameter of any one grain does not exceed the thickness of the membrane structure. The sacrificial layer is removed.
In yet another aspect, the invention features a method of fabricating a membrane filter structure. The method comprises forming a sacrificial layer over a first surface of a substrate. A silicon film is grown over the sacrificial layer at a temperature near the tensile-to-compressive transition temperature of the film such that the silicon film has a grain structure including grains defining pores therebetween wherein the maximum diameter of any one grain does not exceed the thickness of the membrane filter structure. The sacrificial layer is removed.
Various implementations of the invention may include one or more of the following features. The silicon film is formed under a near zero-stress condition. The silicon film has a residual stress within the range of about xe2x88x9250 to 50 mega-Pascals or between about xe2x88x92100 to 100 mega-Pascals. The silicon film is grown such that a lateral dimension of any pore is less than that of any grain. The silicon film is grown such that a lateral dimension of the pores is between about 10 and 50 nm. The silicon film is grown such that the thickness of the film is between about 50 and 150 nm. The silicon film is grown such that a roughness of the film is approximately equal to its thickness. The method may further include forming a conformal layer on the silicon film to provide a selected chemical or biological function. The method may also include monitoring the residual stress of the silicon film.
An advantage of the invention is that it provides a single step method for making a thin film filter membrane. The filter membrane contains nanoscale pores that allow gases, liquids and dissolved molecules to pass though the membrane while preventing a larger species from passing. The pores form as a result of the controlled grain structure of the membrane and no additional processing is required to form the pores. Removal of underlying material is easily achieved by immersing in an etching solution, or by other well-known etching techniques, yielding a free-standing membrane structure.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and in the description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims.